Acetylene is a highly combustible gas composed of carbon and hydrogen. It is used as a fuel gas in the oxyacetylene welding process. An alternate definition is a fuel gas comprising two carbon atoms and two hydrogen atoms, C2H2. If acetylene is compressed on its own, it can lead to a highly exothermic reaction. For this reason, acetylene is supplied dissolved in acetone. Acetylene cylinders are made of a porous material capable of storing the acetone-acetylene solution.